1. FIELD OF THE INVENTION
The invention relates to regeneration of catalytic cracking catalyst.
2. DESCRIPTION OF RELATED ART
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425C-600C, usually 46OC-560C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500C-900C, usually 600C-750C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in many new units because it operates with less catalyst inventory (and hence less catalyst loss and lower catalyst makeup), and tends to have both less CO emissions and less NO.sub.x emissions than the single dense bed regenerators.
Unfortunately, it has not been economically justifiable to convert older style, single dense bed regenerators to the modern H.E.R. design because of the high capital cost associated with simply scrapping the old single bed regenerator. Attempts to use the old regenerator as part of a modern two stage, H.E.R. design have not been too successful, as the old single stage units are larger than either of the beds in an H.E.R. unit.
Rather than scrap older FCC regenerators, refiners have tried to solve some of the problems associated with such units. Poor flow patterns and stagnant regions are problems in themselves, and make worse the NO.sub.x problems associated with bubbling dense bed regenerators. Attempts to improve flow patterns in bubbling dense bed regenerators, and generic attempts to deal with NOX emissions in any type of FCC regenerator will be reviewed below.